Method of blow-molding and blow-molded product

ABSTRACT

A method of blow-molding comprises forming a parison of a thermoplastics material, closing a mold around the parison, injecting compressed gas into the parison through a blowing hole to form a molded product that is shaped according to the shape of the mold, then deforming a seal forming portion of the molded product with a heated sealing tool to seal the blowing hole. The seal-forming portion preferably comprises a tubular collar that surrounds the blowing hole.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of blow molding and to a blow-molded product.

2. Description of the Related Art

Blow-molding processes are commonly used for manufacturing a wide variety of hollow plastics products including, for example, bottles and containers. Typically, a tube (or parison) is extruded into a mold, which is then closed around the parison. A needle is then inserted into the parison and compressed air is injected to blow the parison into the shape of the mold. The needle is then withdrawn, the mold is opened and the finished product is removed.

Products made by the above process are left with a blowing hole formed by the needle, which typically has a diameter of approximately 5 mm. For certain products such as food containers, this blowing hole may have to be sealed to prevent water and dirt from entering and leaving the interior of the molding, as this could contaminate foodstuffs within the container.

An example of a prior art blow-molded product made by the process described above is shown in FIGS. 1 and 2 of the drawings. The product 2 is a wall panel of a bulk container and the drawings show a portion of the wall panel, partially broken away. The panel includes two parallel rectangular side plates 4 that are interconnected by a peripheral wall 6. A shallow recess 8 is provided in one portion of the peripheral wall and a blowing hole 10 is located in that recess. To seal the blowing hole, a plug 12 is inserted and fixed in position. The recess allows the top of the plug 12 to lie flush with the edge of the panel, as shown in FIG. 2.

Inserting a plug to seal the blowing hole in a blow-molded product is a difficult and time-consuming process, which requires an additional component and adds to the overall production costs. There is also a risk that the blowing hole could be incompletely sealed and/or that the plug could work loose during use. It is an object of the present invention to provide a method of sealing the blowing hole in a blow-molded product that mitigates at some of the aforesaid disadvantages.

SUMMARY OF THE INVENTION

According to the present invention there is provided a method of blow-molding, the method including forming a parison of a thermoplastics material, closing a mold around the parison, injecting compressed gas into the parison through a blowing hole to form a molded product that is shaped according to the shape of the mold, and deforming a seal forming portion of the molded product with a heated sealing tool to seal the blowing hole.

The invention allows the blowing hole to be sealed quickly and easily, without requiring an additional component. Production costs are thereby reduced. There is also a reduced risk of an incomplete seal and, because the seal is an integral part of the molded product, it cannot work loose.

Advantageously, the mold is shaped to provide the molded product with a seal-forming portion, which is preferably located adjacent the blowing hole. The seal-forming portion may include a tubular collar that surrounds the blowing hole and extends outwards from the molded product. The tubular collar may for example be molded between an orifice in the mold and a gas injection needle that is inserted into the mold.

The mold is preferably shaped to form a locking element that restricts movement between the tubular collar and the mold. The locking element may include a flange element that is provided on the tubular collar. This helps to prevent the tubular collar being pushed into the molded product when the gas injection needle is inserted.

Advantageously, the mold is shaped to provide the molded product with a recess in the vicinity of the blowing hole and the seal forming portion is deformed to form a seal element that is located within the recess. This construction makes it possible to avoid having a seal element that protrudes outwards beyond the edges of the molded product.

According to a further aspect of the invention there is provided a blow-molded product including a hollow molded body portion and a seal element that seals a blowing hole in the body portion, wherein the body portion and the seal element are made of a thermoplastics material and the seal element comprises a deformed seal forming portion of the body portion.

The blow-molded product can be sealed quickly and easily, without requiring an additional component, with reduced production costs. The risk of an incomplete seal or a loose plug is significantly reduced.

Advantageously, the seal-forming portion is located within a recess provided in the molded body portion.

The blow-molded product may for example comprise a wall panel of a container, or any other suitable product.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described by way of example, with reference to the accompanying drawings.

FIG. 1 is a broken away partial front elevation of a prior art molded product, illustrating the insertion of the sealing plug.

FIG. 2 is a broken away partial front elevation of the prior art molded product, after insertion of the sealing plug.

FIG. 3 is a front elevation of one half of a blow-molding tool according to an embodiment of the invention.

FIG. 4 is a side section showing an extruded parison located between two halves of a molding tool according to the invention, with the mold in an open position.

FIG. 5 is a side section showing the parison within the closed molding tool.

FIG. 6 is a side section showing the molded product after blowing (for clarity, the molding tool has been omitted).

FIG. 7 is a side section of the molded product after removal from the mold, during a sealing operation.

FIG. 8 is side section of the finished molded product after sealing.

FIG. 9 is a sectional front elevation of the finished molded product after sealing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiment of the invention depicted in FIGS. 3 to 9 of the drawings illustrates a method of molding a blow-molded product 14 such as a simple rectangular wall panel, which may for example form part of a bulk container or crate. The wall panel, which is shown in FIGS. 8 and 9, includes two parallel rectangular side plates 16 that are interconnected by a peripheral wall 18. It will be appreciated that many other products may also be made by substantially similar processes and that the present application is intended to cover such products and the processes for making them.

The blow-molding tool 20 shown in FIGS. 3 and 4 includes two symmetrically identical mold halves 22, each of which includes a mating surface 24 that in use mates against a corresponding surface on the other mold half, and a substantially rectangular mold cavity 26 that comprises a side face 28 for molding a side plate 16 of the blow-molded product and a surrounding peripheral face 30 for forming the peripheral wall 18 of the product. The peripheral face 30 includes a portion 32 that protrudes inwards, to form a recess 34 in the peripheral wall of the molded product.

A channel 36 is provided in the mating surface 24 of each mold half 22, at the location of the protruding portion. When the mold halves 22 are brought together, these channels 36 form an orifice 38 that extends through the mold tool 20 from the exterior to the interior of the mold cavity 26. This allows an air injection needle 40 to be inserted through the mold tool 20 into the mold cavity, for blowing compressed air into the parison, to inflate the molded product 14 within the mold.

The features of the blow-molding tool 20 as described above are all conventional. Where the tool differs from prior art blow-molding tools is in the shape of the orifice 38. In the prior art, this orifice generally consists simply of a cylindrical bore having a diameter slightly greater than that of the air injection needle 40, so as to allow the needle to be inserted. In the embodiment of the invention shown in the drawings, the orifice 38 includes an outer portion 38 a comprising a cylindrical bore with a diameter slightly greater than that of the needle 40 (for example approximately 5 mm), an inner portion 38 b of slightly larger diameter (for example approximately 8 mm) and an intermediate portion 38 c of even larger diameter (for example approximately 10 mm). It will be appreciated that the dimensions quoted above are only illustrative and are not intended to be limiting in any way.

The inner and intermediate portions 38 b, 38 c of the orifice are therefore significantly larger than the needle 40, thus forming an annular gap between the needle 40 and the sides of the orifice 38. During use, melted thermoplastics material is molded in this gap to form a tubular collar, which is subsequently deformed to seal the blowing hole.

The blow molding process will now be described in detail with reference to FIGS. 4-7.

First, as shown in FIG. 4, a tube of heated thermoplastics material is extruded through an extrusion nozzle 42 into a gap between the two halves 22 of the mold, which at this stage is open. The extruded material forms a parison 44 comprising a hollow tube that is closed at its lower end.

The mold 20 is then closed as shown in FIG. 5. The upper part of the parison 44 is nipped between the two halves of the mold forming a plug of material within the orifice 38 in the upper part of the mold. The air injection needle 40 is then inserted through the orifice 38 into the parison 44 and compressed air is injected to inflate the parison, so forming a molded product 14 matching the internal shape of the mold, as shown in FIG. 6 (the mold tool has been omitted in this drawing for the sake of clarity. Once the thermoplastics material has set, the needle 40 is withdrawn, leaving a blowing hole 45. The mold 20 is then opened and the molded product 14 is removed.

As can be seen in FIGS. 6 and 7, the molded product 14 includes a tubular collar 46 that extends from the upper edge of the molded product, this collar having been shaped within the gap between the wall of the orifice 38 and the air injection needle 40. The lower part of the collar 46 is substantially cylindrical, having been formed by the inner portion 38 b of the orifice, and at its upper end the collar has an outwardly extending flange 48 formed by the intermediate portion 38 c of the orifice. The collar 46 and the flange 48 provide a seal forming portion 49 of the molded product, which may be deformed to seal the blowing hole 45.

After the blow-molded product has been removed from the mold tool, it is subjected to a second processing step, in which a heated sealing tool 50 is applied to the seal-forming portion 49. The sealing tool 50 re-heats and melts the thermoplastics material of the seal-forming portion 49 and squashes it downwards, thereby sealing the blowing hole 45. The remelted material thus forms a dome-like seal element 52 that is accommodated within the recess 34 in the edge of the molded product 14, the upper part of the seal element 52 lying flush with or slightly below the edge of the product, as shown in FIGS. 8 and 9.

The process described above thus provides a very simple method for sealing the blowing hole 45. The seal element 52 is formed as an integral part of the blow-molded product 14, thereby obviating the need for a separate plug. Sealing the blowing hole 52 is a very simple process involving the application of a heated sealing tool 50. This process can be carried out manually or it can be automated, if desired.

The shape of the seal-forming portion 49 is important for successful sealing of the product. We have found that the seal-forming portion 49 preferably comprises a tubular collar 46 having a flange 48 at its outer end. The flange 48 supports the collar 46 within the orifice 38 in the mold 20 and prevents it from being deformed as the needle 40 is inserted. We have found that if a flange 48 is not provided, the collar 46 tends to be forced inwards as the needle 40 is inserted, which makes subsequent sealing very difficult.

It will be appreciated that various modifications of the process described herein may be made, the process described being merely illustrative of one embodiment of the invention. 

1. A method of blow-molding, the method comprising: forming a parison of a thermoplastics material, closing a mold around the parison, injecting compressed gas into the parison through a blowing hole to form a molded product that is shaped according to the shape of the mold, and deforming a seal forming portion of the molded product with a heated sealing tool to seal the blowing hole.
 2. A method according to claim 1, wherein the mold is shaped to provide the molded product with a seal forming portion.
 3. A method according to claim 2, wherein the mold is shaped such that the seal-forming portion is located adjacent the blowing hole.
 4. A method according to claim 2, wherein the seal forming portion includes a tubular collar that surrounds the blowing hole.
 5. A method according to claim 4, wherein the tubular collar extends outwards from the molded product.
 6. A method according to claim 4, wherein the mold is shaped to form a locking element that restricts movement between the tubular collar and the mold.
 7. A method according to claim 6, wherein the locking element includes a flange element that is provided on the tubular collar.
 8. A method according to claim 1, wherein the mold is shaped to provide the molded product with a recess in the vicinity of the blowing hole.
 9. A method according to claim 8, wherein the seal forming portion is deformed to form a seal element that is located within the recess.
 10. A blow-molded product comprising a hollow molded body portion and a seal element that seals a blowing hole in the body portion, wherein the body portion and the seal element are made of a thermoplastics material and the seal element comprises a deformed seal forming portion of the body portion.
 11. A blow-molded product according to claim 10, wherein the seal-forming portion is located within a recess provided in the molded body portion.
 12. A blow-molded product according to claim 10, wherein the blow-molded product comprises a wall panel of a container. 